This invention relates to a photonic switch used in an optical communication system and, more particularly, to a photonic switch that uses a micro mirror manufactured utilizing micro machine technology.
The increase in the speed and capacity of optical networks has been accompanied by demand for photonic switches that are capable of switching a circuit path using an optical signal as is without converting it to an electrical signal. Though a wide variety of such switches are in use and depend upon the number of switched channels, a two-dimensional photonic switch that uses MEMS (Micro-Electro Mechanical Systems) technology is considered to be well-suited for an intermediate number of channels on the order of 4 to 32 channels. FIG. 10 illustrates the structure of a 4-channel photonic switch serving as an example of a two-dimensional MEMS photonic switch. The photonic switch comprises optical fibers FB11 to FB14 of input ports (input channels) #1 to #4, lenses LS11 to LS14, optical fibers FB01 to FB04 of output ports (output channels) #1 to #4, lenses LS01 to LS04 and a plurality of micro MEMS mirrors MM11 to MM44 fabricated on a silicon substrate by a semiconductor manufacturing technique.
Light that has propagated through the fibers FB11 to FB14 is converted by the lenses LS11 to LS14 to collimated light suitable for transmission through space. The MEMS mirrors MM11 to MM44 can be erected by the action of static electricity. The photonic switch is such that erecting specific mirrors causes light that has propagated through the fibers FB11 to FB14 to be reflected, thereby enabling the optical paths to be switched to the positions of desired fibers on the output side. The bold lines in FIG. 10 indicate MEMS mirrors that have been erected, while the fine lines indicate MEMS mirror that are in a reclining attitude. Accordingly, in FIG. 10, MEMS mirrors MM1, MM32 and MM44 are upright and, as a result, optical signals from input channels #1, #3 and #4 have been cross-connected to output channels #1, #2 and #4, respectively.
FIG. 11 is a schematic view useful in describing the erection of a MEMS mirror, in which (A) is a plan view and (B) a side view. A mirror plate MPL on which a circular switching mirror MR has been formed is rotatably mounted on a silicon substrate SBD by a hinge HG. A translation plate TPL is translated to the left and right by a scratch drive actuator SDA and is connected to the mirror plate MPL by push rods PR1, PR2 via hinge joints HGJ. The mechanism further includes springs SP and holding plates HPL. If a voltage of ±100 V is applied to the scratch drive actuator SDA at a frequency of 500 kHz, the translation plate TPL will move rightward, enabling the mirror plate MPL to be erected to an angle of 90° in 0.5 ms. If drive is removed, the translation plate TPL is moved leftward by the springs SP, whereby the plate can be reclined instantaneously.
It should be obvious from FIG. 10 that the optical path lengths of this two-dimensional MEMS photonic switch differ depending upon the path through the switch. In the case of FIG. 10, the optical path of longest length is the path that extends from input channel #1 to output channel #1 and has a length of 2b+6a. The optical path of shortest length is the path that extends from input channel #4 to output channel #4 and has a length of 2b. Here b represents the distance from the leading end of the fiber to the MEMS mirror, and a represents the distance between mutually adjacent MEMS mirrors. This difference in optical path length gives rise to a variation in loss from path to path.
FIG. 12 is a diagram useful in describing loss ascribable to a difference in optical path length. Assume that an optical system, namely the focal lengths of lenses LSI, LS0 and the distances between fibers FBI, FB0 and the lenses LSI, LS0, respectively, has been designed so as to minimize loss over an optical path length D, as shown in (B) of FIG. 12. If the length of the optical path becomes shorter than D, as shown in (A) of FIG. 12, a light beam BM enters the lens LS0 in a form narrower than in the case of (B). As a result, the light beam is narrowed down to the maximum extent at a position a distance d in front of the fiber FB0 and then enters the fiber FB0 while its diameter increases. If the beam diameter when the beam enters the fiber FB0 is greater than the inner diameter of the fiber FB0, loss occurs. On the other hand, even if the beam diameter is smaller than the inner diameter of the fiber FB0, narrowing down the beam too much when it enters the fiber will cause loss because the beam will not be guided through the fiber appropriately. Further, if the length of the optical path becomes greater than D, as shown in (C) of FIG. 12, the beam BM will spread too much at the position of the lens LS0 and loss will be the result. In addition, since the beam is narrowed down to the maximum extent at a position d rearward of the fiber end, not all of the light from the lens LS0 enters the fiber FB0 and, hence, loss is produced.
Variation in loss in photonic switches is a problem in terms of system application and therefore methods of reducing such variation in loss are being studied. Since a variation in loss essentially is caused by path dependency of the optical path length, a conventional method disclosed in the specification of Japanese Patent Application Laid-Open No. H13-59089 seeks to reduce variations in loss by making optical path length uniform. FIG. 13 is a diagram showing the structure of a photonic switch according to the prior art, in which components identical with those of the photonic switch shown in FIG. 10 are designated by like reference characters. This switch differs in that (1) fixed mirrors FMR1, FMR2 are provided in addition to the MEMS mirrors and the optical path lengths between channels are rendered uniform by using the fixed mirrors to reflect light; (2) the optical fibers are disposed at oblique angles; and (3) the MEMS mirrors that are erected in accordance with the path differ from those erected in FIG. 10. The bold lines in FIG. 10 indicate MEMS mirrors that have been erected, while the fine lines indicate MEMS mirrors that are in a reclining attitude. The bold lines in FIG. 13 indicate MEMS mirrors that have been erected, while the fine lines indicate MEMS mirrors that are in a reclining attitude. Four paths, namely #1→190 1, #2→#2, #3→#3 and #4→#4, are illustrated in FIG. 13. All of these paths have the same optical path length.
In this example of the prior art, it is required that the fibers and lenses (not shown) be arranged in staircase-fashion, as will be understood from FIG. 13. FIG. 14 is an external view of the end of a fiber array used in the photonic switch of FIG. 13. This shows that the end face of the array must have a staircase shape. The end face of fibers is jagged and light will not emerge in a straight line under these conditions. This necessitates grinding and polishing of the surface. With the photonic switch of the prior-art example shown in FIG. 10, the end face of the fiber array can be made flat. This means that grinding and polishing is easy to perform and manufacture is easy as well. With the fiber array of FIG. 14, on the other hand, the end face has a step-like configuration. This makes grinding and polishing and, hence, manufacture difficult.
Further, with the photonic switch of FIG. 13, planar lenses cannot be applied. This switch requires that spherical or rod lenses be arranged in highly precise fashion in staircase fashion. Problems arise in terms of mounting precision and cost.
Furthermore, with the photonic switch of FIG. 13, the additional fixed mirrors are required. This leads to problems in terms of reducing size and simplifying manufacture.